This invention relates generally to new ester compounds, liquid crystal compositions including at least one ester compound and to a method of preparing the ester compounds, and particularly to ester compounds which increase the absolute value of the dielectric anisotropy in the high frequency range in a liquid crystal composition suitable for use in a two frequency matrix-addressing drive system.
Liquid crystal display devices are now being widely used for digital displays in electronic devices, such as calculators, timepieces and the like. The method conventionally used for driving the liquid crystal display has progressed from the conventional static drive mode to a multiplex drive mode. The multiplex drive mode now being used is generally referred to as the AC amplitude selective multiplexing method. As long as this method is employed, the number of rows which may be driven is four at the most.
In the generalized AC amplitude selective multiplex method, the ratio V.sub.on /V.sub.off of the effective voltage V.sub.on of the signal applied to the lighted picture cell to that voltage V.sub.off applied to the non-lighted picture cell is determined by the number of multiplex lines N. The value of the ratio can be no more than the following formula: ##EQU1## even in the most stable of conditions.
In view of this relationship of the voltage ratio V.sub.on /V.sub.off, the value decreases with an increase in the number of multiplex lines. In other words, as the number N increases, a deterioration in contrast occurs. In presenting a complex character display or a graphical display, it is necessary to increase the number N. Thus, there has been difficulty in applying the conventional generalized AC amplitude selective multiplexing drive method to this type of character or graphic display. Recently, a two-frequency matrix-addressing mode has been found to be effective. The two-frequency mode takes advantage of the low frequency dielectric relaxation phenomenon in a low frequency range and the display is driven by two frequencies. It is expected that this two-frequency matrix-addressing mode can be effective for such complex displays.
The two-frequency matrix-addressing method suffers from the shortcoming that energy consumption is high when the multiplex matrix is addressed. This is due to the fact that high frequencies are applied and the applied voltage increases. This energy consumption may be effectively reduced by making the driving voltage smaller. It is known that the driving voltage V is dependent upon the dielectric anisotropy liquid crystal .DELTA..epsilon.. This relationship may be defined as follows: ##EQU2## In other words, as the absolute value .vertline..DELTA..epsilon..vertline. increases, the value of driving voltage V may be reduced.
The two-frequency matrix-addressing method is characterized in that the voltage ratio between the lighted condition and the non-lighted condition depends not only on the number of rows to be driven, but also the driving voltage and dielectric content as noted in equation (B). In order to take advantage of this relationship, a display device includes a liquid crystal composition wherein the dielectric anisotropy changes from positive to negative depending on this dielectric relaxation as the frequency increases. Such a display device will provide a display without contrast deterioration due to an increment in the number of multiplex lines as it is driven by signals of different frequencies. The first of these signals utilize the positive dielectric anisotrophy of the liquid crystal and the other frequency utilizes the negative dielectric anisotropy of the liquid crystal material.
It is believed that this occurs because the voltage ratio V.sub.on /V.sub.off of the effective voltage V.sub.on of the signal applied to the lighted picture cell to the effective voltage V.sub.off of the signal applied to the non-lighted picture cell is dependent not only on the multiplex line number N, but also to the peak value of the voltage of the applied voltage and the dielectric anisotropy of the liquid crystal. Thus, the effective voltage ratio V.sub.on /V.sub.off can be increased by increasing the peak value of the voltage of the applied signal in the case of the two-frequency method. This permits a display device to provide a display having improved contrast, even when the number of lines N is increased in the two-frequency method. This particular characteristic distinguishes the two-frequency matrix-addressing method from the generalized AC amplitude selective multiplexing method. Specifically, in the latter the voltage ratio V.sub.on /V.sub.off is determined only by the number of lines N, as shown in formula (A).
Liquid crystal material utilized in the two-frequency matrix-addressing mode has a frequency-dependent dielectric anisotropy which is positive at low frequencies and negative at high frequencies and permits a decrease in driving voltage. This inversion of dielectric anisotropy occurs about the critical frequency F.sub.c at which point the dielectric anisotropy is 0. In order to take advantage of the effects noted above, it is essential that the dielectric anisotropy of the liquid crystal material by highly negative at high frequencies. Specifically, the absolute value of the dielectric anisotropy should be as large as possible.
Accordingly, it is desirable to provide a liquid crystal composition which will have the desired characteristics. Such a liquid crystal compositions may be obtained by mixing a nonliquid crystal materials with a liquid crystal composition, particularly one having a negative dielectric anisotropy at high frequencies. Non-liquid crystal compounds which can be mixed with liquid crystal materials have not been known which will reduce the driving voltage V. Accordingly, it is desirable to provide such a compound, liquid crystal composition including such compound and a method of preparing the compound.